1620 PHOTOCHEMISTRY OF CHLOROPHYLL CHAP. 35 



Quantum Yield. Figure 35.26 shows a lower yield of quinone reduction 

 compared to photosynthesis in Chlorella in subsaturating light. This 

 seemed to point to a lower maximum quantum yield of the former reaction ; 

 Clendenning and Ehrmantraut (1951) found, in fact, about a 70% lower 

 efficiency in the weakest light used. However, the percentual difference 

 between the rates of the two reactions seems to have a maximum in the 

 region of half-saturation, and to decline in stronger and in weaker light. 

 Ehrmantraut and Rabino witch (1952) found that this difference disappears 

 in both limiting cases — that of light saturation as well as that of light limi- 

 tation. Their quantum yield measurements, carried out partly with filtered 

 neon light and a Warburg-Schocken actinometer and partly with a mono- 

 chromator and bolometer, were summarized on pages 1130-1131. The 

 second method, which is more precise, gave an average quantum efficiency 

 of 7 = 10 ± 1, practically identical with that found in parallel experiments 

 for the quantum efficiency of the photosynthesis of the same cells in carbon- 

 ate buffer No. 9. The actinometric data were slightly, but not significantly 

 higher (cf. table 29.XI). 



These results, together with those obtained with chloroplast suspensions, 

 support strongly the hypothesis that the primary photochemical process is 

 the same in photosynthesis and Hill reaction and requires the same number 

 of quanta for the transfer of one hydrogen atom, despite the fact that much 

 less energy is needed to transfer it to a typical Hill oxidant, than to carbon 

 dioxide. 



The relation between the light curves of photosynthesis and those of the 

 Hill reaction in Chlorella — convergence in two limits, divergence in the mid- 

 dle — is peculiar, and cannot be explained by a simple kinetic model. It is 

 probably related to the "self-poisoning" of the Hill reaction by quinone, 

 which may leave the saturation rate unchanged, and the effect of which 

 on the rate in subsaturating light may depend on the duration and intensity 

 of illumination (so that in measurements in weak light, self-poisoning has 

 no time to develop itself). 



Yield in Flashing Light. The experiments on the oxygen liberation by 

 Chlorella cells in quinone in flashing light already were described in chapter 

 34 (section B7). As illustrated by figure 34.26 (Clendenning and Ehrman- 

 traut 1951), the dark intervals needed to obtain the maximum yield per 

 flash (with "instantaneous" discharge flashes) were found to be the same for 

 quinone reduction and for photosynthesis of the same cells in bicarbonate. 

 Experiments with increased flash energy by Ehrmantraut and Rabino- 

 witch (1952) indicated (figure 34.27) that the maximum yield per flash 

 also is the same for both reactions, although it requires a higher flash 

 energy in the case of quinone as oxidant (the same relation was noted also 

 in continuous light, cf. figure 35.26). The significance of these findings — 



